It is known from DE 44 25 207 A to move a welding jaw of a transverse sealing station of a vertical tubular bag machine along a closed path. In this case, the path has a straight portion, which is provided for making contact with a film tube pressed together between the welding jaw and a second, oppositely running welding jaw. In this case, the film tube and the welding jaw are moved further at the same speed in the direction of the straight portion, in order to weld the film tube by exposure to the heat of the welding jaws within a sealing time. Two drives, acting in directions perpendicular to each other and moving the welding jaw, serve for producing a circulating movement of the welding jaw, a complete jaw circulation being achieved within a cycle time.
The known method has the disadvantage that, for a specific bag length, it is always definitively prescribed what percentage of the cycle time, i.e. the time for a 360° circulation of the welding jaw, is available for the time in which the sealing takes place. This sealing time, which generally comprises the period of time during which the welding jaw is in contact with the film tube, that is to say is welding it, is sometimes also defined for mechanical engineering reasons in such a way that it comprises both the period of time during which the welding jaw is positioned at a slight distance away from the film tube, in order to preheat it or cool it by means of cooling air (after welding has taken place), and the period of time of pressing the welding jaw onto the film tube along an effective sealing path (Zseff).
Since the percentage of the cycle time intended for the sealing time (however it is defined) is always constant, bags of any length are welded with a fixed percentage, which is referred to as the sealing time factor. Similarly, film types are always welded with a specific sealing time factor, irrespective of their thickness and their structure (layers, types of plastic, layer thicknesses). The sealing time factor is in this case kept constant in the same way, irrespective of a preselected bag length, since in the known prior art coupling elements between two drives acting perpendicularly to each other provide for this.
Also, in the case of the known methods, the sealing time factor is kept constant for a constant bag length if there is a change of film type. This has the disadvantage that different film types are welded with different quality, in particular at relatively high output. Certain film types which are subjected to the constant percentage of the cycle time as a sealing time at high output, i.e. with a low cycle time, are no longer reliably welded if they in fact require a relatively great amount of heat to be introduced as a result of their great thickness or their multilayered construction. Although this can be technically remedied in terms of the gearing by using a different coupling or cranking technique, if it is subsequently intended again to weld a film type that requires a smaller amount of heat to be introduced or a shorter bag length is to be produced, the inflexible, greater sealing time factor has the effect that the packaging output is reduced, since too much sealing time is provided, which then prolongs the cycle time. The longer the cycle time is, the fewer welding operations can be carried out per minute. For a cycle time of 1 sec, 60 tubular bags per minute are transversely welded, and consequently produced, on a vertical tubular bag machine.